The invention concerns in particular a cardiac pacemaker as is usually employed to ensure by means of electrical pulses to the myocardium of a heart a cardiac rhythm which does justice to the hemodynamic demand of a patient. Such pacemakers are usually connected by way of an electrode line to electrodes which are suitable for intracardial arrangement and for electrical stimulation of the heart tissue (myocardium) by delivering electrical pulses to the myocardium. As such pacemakers are usually implanted, it is a matter of particular significance that the amount of energy delivered with an electrical stimulation pulse is precisely such that the heart tissue is safely and reliably stimulated. A stimulation pulse which is excessively rich in energy would excessively rapidly drain the battery of the pacemaker. On the other hand, a stimulation pulse which was too low in energy would possibly not suffice to excite the heart tissue in the sense of a stimulus which appropriately propagates and results in contraction of a corresponding chamber of the heart.
With that background in mind it is known, in conjunction with the delivery of a stimulation pulse to the myocardium, to implement a check in respect of the success of stimulation by the evaluation of electrical signals which occur in conjunction with the delivery of the stimulation pulse, in order for example in the case of an absence of stimulation success to be able to trigger a backup stimulation pulse with a higher energy content. Stimulation success monitoring presupposes the detection of successful stimulation which is also known as capture recognition. Numerous devices and methods are known which are intended to permit capture recognition.
A large number of systems for automatic capture detection have already been developed, which in connection with cardiac pacemakers serve to detect the presence/absence of depolarisation of the myocardium following a stimulation pulse. Those systems usually employ very simple methods such as evaluation of the signal amplitude—that is to say, the maximum amount of the signal—in order to detect myocardium depolarisation which characterises stimulation success, and in that way to distinguish same from signals which are involved with a lack of stimulation success. Such systems operate satisfactorily as long as the polarisation voltage is low and a typical signal which characterises stimulation success occurs. However, the result of the fusion of stimulated and natural cardiac reactions (a fusion event) can be that the signal following a stimulation pulse is atypical and is attributed by a simple algorithm to a lack of stimulation success. In addition, strong polarisation voltages can simulate a signal which is characteristic of successful stimulation. There is therefore a problem in reliably detecting the electrical signals evoked in the myocardium in the presence of such signal artefacts which are due for example to polarisation effects in the region of the interface between an electrode and body fluid or myocardium.
Various ways of resolving the last-mentioned problem are proposed for example in U.S. Pat. No. 5,417,718 to Kleks (23 May 1995), U.S. Pat. No. 5,697,957 to Noren (16 Dec. 1997), U.S. Pat. No. 5,861,013 to Peck (19 Jan. 1999), U.S. Pat. No. 5,873,898 to Hemming (23 Feb. 1999, and U.S. Pat. No. 5,941,903 to Zhu (24 Aug. 1999), as well as European patent application EP 0 826 392, also to Noren.
It is also known to detect characteristic evoked signals on the basis of typical signal shapes, for a positive stimulation success.
Unfortunately recognition of stimulation success is tainted in all cases with statistical uncertainties so that there is still a wish to implement capture recognition with a higher level of specificity and sensitivity.